(i) Field of the Invention
This invention relates to improvements in the drilling of oil and gas wells in polar regions. More particularly, it relates to improved techniques for effectuating such drilling in the wintertime in the Arctic Ocean and more especially in the Beaufort Sea area of the Arctic Ocean, although it is feasible for application in other areas where similar conditions exist. More specifically, it relates to method and apparatus for sealing of a water/air interface around canopy systems for ensuring the provision of an ice-free zone around a drillship to enable such wintertime drilling.
(ii) Description of the Prior Art
At the present time drilling in offshore Arctic regions is carried out in the summertime either by the use of drillships anchored at a drill site where the risk of impingement by ice floes is minimal, or through the use of artificial islands. Summertime drilling is feasible for depths from 60 feet to 200 feet or more. Artificial islands currently being used in the shallow water regions of the Beaufort Sea become excessively expensive in water depths of 40 feet or greater. It appears uneconomical at the present time to build artificial islands for exploratory drilling wells in water depths exceeding 40 feet. It may be economical to drill production wells from artificial platforms in water depths exceeding 40 feet. Moreover, it is presently not feasible to drill exploratory wells from floating ice islands in the regions where ice movement is too great (i.e., more than a few feet).
One of the chief obstacles to overcome in drilling in Arctic regions is the Arctic pack ice. The ice grows to a thickness of approximately 6 feet and is laced with pressure ridges and ice islands which can reach thicknesses of over 100 feet. The pack ice moves at speeds from 0 to 20 or more miles per day with an average movement of approximately 2 miles per day.
If drilling were to take place in waters where there was considerable ice movement, a very solid structure would be required in order to withstand the forces exerted upon it by the ice pack and yet to be able to remain on position in order to drill a well. For exploratory drilling operations, a solid bottom founded structure should be provided which could resist the movement of the ice pack and yet would be mobile enough to be transported from one exploratory drilling site to another.
Drilling with structures on the sea bottom has numerous problems. Firstly, there is the problem of designing hulls which could withstand the ice forces from deep ice keels against these structures. In the second place, subsea systems would have problems of buoyancy, lifesupport systems, power supply, and access for crews and maintenance.
It would, therefore, be desirable to develop another technique that would allow exploratory drilling during winter months. Such system should also be able to extend operational capability into the shorefast ice. This technique has the potential for drilling wells more economically than any other method in the shorefast ice regions. Using this technique, the rate of exploration in the Beaufort Sea would be increased by a factor of from two to four times. This technology of using drillships in shorefast ice could be applied to other regions of the Canadian Arctic. The techniques used for drilling in shorefast ice are a logical step toward developing year-round drilling systems in pack ice regions.
The development of such technology is important since the risks to the environment of a drilling system in the shorefast ice are relatively low. One advantage of operating in shorefast ice is that the ice moves very little throughout the winter. Any oil spilled underneath the ice would be confined to a very small area where it could be removed from the environment.
Such a drill system requires an essentially ice-free zone around the ship so that the ice, should it move by up to 30 feet or more, will not bear against the second drillship. A canopy with a warm air system is proposed to provide this substantially ice-free zone.
The initial problem which the present invention proposes to overcome is to assure the sealing of the air/water interface around the canopy system, by a skirt that wil substantially prevent cold air from going under the canopy but not put any substantial forces on the vessel if the ice moves.
One system for providing a substantially ice-free zone around a drillship is disclosed and claimed in copending United States application Ser. No. 54,997 filed on July 5, 1979 (the relevant contents of which are incorporated herein by reference) directed to a method and warm air canopy providing a substantially ice-free zone around a vessel. The method comprises: (a) creating a finite substantially enclosed zone completely around the periphery of the vessel above the waterline, such zone including a peripheral zone approaching the waterline; (b) continuously circulating warm air within the finite substantially enclosed zone; and (c) continuously injecting warm air into such peripheral zone to provide a substantially ice-free peripheral zone.
The warm air canopy referred to above comprises in combination with the vessel: (a) a plurality of floating modules disposed around, and connected at one end to, the vessel; (b) a continuous, downwardly depending skirt extending completely around the outer periphery of the plurality of floating modules; and (c) an air/water heat exchanger in a selected plurality of the floating modules.
A problem which has arisen with the above two systems is that of ensuring a seal around the canopy system which would substantially prevent cold air from going under the canopy but which would not put any substantial forces on the vessel if the ice moves.